1. Field of the Invention
This invention is directed to fabrication methods for producing multi-faceted scanner mirrors, particularly adapted for inner-scanners.
2. Description of the Prior Art
Multi-faceted scanner mirrors have become a predominant requirement for the design and manufacture of a cost-effective electro-optical scan system. One use of such a mirror is shown in Maddox et al U.S. Pat. No. 3,971,917, issued July 27, 1976 for "Labels and Label Readers", showing an outside-faceted scanner mirror for scanning labels.
Basically a multi-faceted scanner mirror consists of different reflecting facets on a rigid unit (such as of copper or beryllium copper), positioned so that the facets collectively define the scene to be scanned. Each facet has a specified pyramidal angle at a given rotational angle, with some facets sometimes formed to interlace with the scan of others.
One of the problems facing production of multi-faceted mirror scanners is to assure that each facet is highly polished and flat, with sufficient reflectance, and to position each facet precisely in relation to the incident light beam, so as to reflect the beam efficiently and precisely toward the predetermined target point. One arrangement for doing so is described in Hug U.S. Pat. No. 3,781,079 issued Dec. 25, 1973, which requires adjusting each facet individually by a separate action to cause the beam to be reflected precisely toward the target point.
Maddox U.S. Pat. No. 3,972,917 shows a faceted mirror in the form of a drum. The mirror forming each of the facets is secured to a flat surface on the circumference of the drum by screws and washers. Each mirror is mounted upon a resiliently depressible adhesive, and the angular relationship of each mirror to its support is adjusted by the mounting screws to distort the depressible adhesive in order to properly orient the facet.
Another prior art procedure which has unsuccessfully attempted to solve the problem of facet orientation is a bonded built-up-facet process by which stainless steel sheet metal pieces are formed into individual facets, which are polished by lapping to provide facets of the required individual size, shape, surface finish and figure. The individual facets are then bonded to a main frame or ring that is formed to hold the individual facets with the required pyramidal and rotational angle dimensions. However, this process suffers from several major disadvantages: among others, it is labor intensive, requiring highly trained precision work; the adhesive for secure bonding requires substantial time for curing; the product is subject to problems of "peel off" during subsequent fabrication or use.
Accordingly, a relatively inexpensive and simple method of fabrication for producing a reliable high precision multi-faceted scanner mirror with excellent reflectance has been required.
Scanner mirrors have been manufactured using either of two known approaches, namely, the "Milling/Single-Point Diamond Flycutting" process and the "Wire EDM/Single-Point Diamond Flycutting" process, as discussed below in greater detail. Each process possesses drawbacks; in particular, use of the single-point diamond flycutting in either process, is relatively expensive in that it requires the use of single-point diamond cutter tools and specialized machine tools to produce the required optical "figure" and high finish. The material usually used (e.g., beryllium copper) causes the diamond cutter to wear very quickly. The single-crystal diamond cutter tool requires unique characteristics: the highest possible hardness, low friction, high stiffness, good thermal conductivity, and an edge that can be sharpened. Because of its sharpness, the diamond cutting edge will transfer any undesired machine motions to the workpiece optical surface. Because of these factors, these specialized diamond tools are not cost effective to produce multi-facet scanners with reasonable yield.
Furthermore, for attaining the required accuracy normally associated with precision machining, the machines must be very stiff, have no lost motion or backlash, have no internal vibrations, be isolated from external vibration, and be thermally stable. In addition, all machine motion must be exceptionally smooth, and the control system must have micro-inch resolution. Such machines must be specially designed and will naturally have a substantially greater cost.
In addition, the width of the facets is limited by a phenomenon ("trapezoidal effect") caused by the single-point diamond flycutter and the geometry of the generated facet. That trapezoidal effect limits the clear aperture of the facets and leaves unfinished the facet-to-facet intersections, which thereby contribute to stray light scatter, not detected by visible inspection but only by system performance and functional tests. The associated expense of such functional tests, required to determine whether to accept or reject the workpiece, is generally considerable. Acceptance only after system testing for such tests usually restricts the yield and thereby disadvantageously requires an excessively large number of scanners to be manufactured and results in costly wastage of completed units. Moreover, a substantial amount of time, typically on the order of at least twelve hours, is required to produce a scanner using the "milling/single-point diamond flycutting" process. Notwithstanding its disadvantages of high cost and restricted potential for quantity production, inner-scanner manufacturers thus far have had to rely primarily on the single-point diamond flycutting process.
Other processes have been evaluated without success, including in particular, the bonded, built-up facet process mentioned above. While this process eliminates the single-point diamond flycutter operation, it is excessively labor intensive; this problem is particularly acute whenever one or more individual facets needs to be reworked. Moreover, the adhesive between the main frame and the facet requires substantial time to cure, on the order of eight to twelve hours. Once the adhesive is cured poor adhesion and potential "peel off" occurrences during subsequent manufacturing procedures and/or uses of the scanner cause problems. Several attempts have been made to promote the required adhesion; however, these attempts either result in only a marginal improvement in adhesion or are too expensive or complex to implement in practice. An inexpensive and simple solution has yet to be found. As a result, considerable time and expense are often disadvantageously consumed in fabricating a reliable inner-scanner by this bonded built-up facet process.